A device for displaying a color image such as a projector device or a projection TV requires light sources of three colors of R (red), G (green), and B (blue) as light sources. There is a case in which a semiconductor laser element with a high luminous efficiency is used as the light source. As one of characteristics of a semiconductor laser, it is specified that its output light is coherent in phase, in other words, a high coherence; however, due to this, there arises a problem of appearance of a speckle pattern on a projected surface, a so-called speckle noise.
Thus, as one method of reducing the speckle noise, a means is adopted that a plurality of wavelengths are mixed to reduce the coherence.
Conventionally, as such a device for oscillating the plurality of wavelengths at the same time to reduce the speckle noise, there is the one including: a semiconductor laser for generating a laser beam with a wavelength λ1, and a semiconductor laser for generating a laser beam with a wavelength λ2 different from the wavelength λ1. Since the wavelength λ1 and wavelength λ2 are different from each other by about several nm to several ten nm, it is difficult to visually distinguish the difference in wavelength between the two beams, and hence the two beams look like the one of the same color. On the other hand, reduction of the coherence can obtain an effect of reducing the speckle noise.
Further, as a method for obtaining the plurality of wavelengths, there is also a method in which a plurality of active layers are crystal grown on one substrate as shown in Patent Document 1 or Patent Document 2.